Fast-neutron nuclear reactors include a core constituted by fuel assemblies, in which core heat is released due to the effect of the nuclear reactions.
The fuel core, which is placed inside a large-sized vessel, is cooled by a heat-transfer fluid which is generally constituted by liquid metal.
In addition, the nuclear reactor includes at least one cooling system in which the heat released by the core and taken up by the heat-transfer liquid metal enables the feed water to be heated up and vaporised inside steam generators.
In general, the heat released by the core is transmitted to the feed water via a primary coolant constituted by a first liquid metal, which may be liquid sodium, circulating inside the reactor vessel, and via a second liquid metal which may also be constituted by sodium and which circulates in the secondary cooling system of the nuclear reactor on which the steam generators are placed.
In the case of integrated-type fast-neutron nuclear reactors, the nuclear-reactor vessel is filled with liquid metal such as sodium in which the reactor core is submerged. Intermediate heat exchangers are also submerged in the liquid metal filling the vessel and each include a heat-exchange element connected to the secondary cooling system of the steam generator and in which the secondary heat-exchange fluid circulates.
The liquid metal filling the vessel and constituting the primary coolant cools the core and yields up the heat taken up from the core assemblies to the secondary coolant circulating in the intermediate heat exchangers.
Inside the steam generators, the secondary coolant heats up and vaporizes the feed water and cools down.
When it is desired to operate the reactor under accident conditions or to shut it down, for example prior to maintenance, repair and/or core-reloading operations, control rods made of neutron-absorbing material are inserted inside some of the core assemblies, constituting the assemblies for controlling the nuclear reaction.
The control rods, in the maximum insertion position inside the core, make it possible for the most part to stop the nuclear reactions occurring in the core and releasing energy.
However, because the materials constituting the core are activated, after a period of operation of the nuclear reactor, the nuclear reactor possesses residual energy which is transmitted to the coolant in the form of heat.
It is necessary to remove this residual energy in the form of heat in order to cool the nuclear reactor completely, prior to carrying out work inside the vessel.
It is known to use heat exchangers designed especially to remove the residual heat of the nuclear reactor at shutdown.
In the case of integrated-type fast-neutron nuclear reactors, these heat exchangers may be submerged in the nuclear-reactor vessel, like the intermediate heat exchangers, so as to cool the primary coolant directly.
Such devices are complex and expensive to produce and require additional coolant-feed systems.